The photoreceptor layer of vertebrate retinas contain high concentrations of cyclic GMP and of enzymes involved in cyclic GMP metabolism. Retinal rod outer segments are modified cilia. We have previously isolated the axoneme and basal apparatus of bovine rods and shown that the isolated material contains tightly bound guanylate cyclase activity. Our goals for the next year are as follows: 1. To separate and study the proteins which are components of the rod axoneme, and if possible to isolate and localize the guanylate cyclase. 2. To continue a study of the enzymology of the guanylate cyclase. We wish specifically to examine the effects of potential activators and inhibitors, such as Ca 2 ion. 3. To examine the activity of other enzymes involved in cyclic GMP metabolism in the retina. We are particularly interested in pyrophosphatase, 5'-GMP phosphatase, guanylate kinase and protein kinase. In addition, we hope to continue our study of cyclic nucleotide involvement in the light response of the crayfish caudal photoreceptor unit.